1. Field of the Invention
The present invention relates to a high efficiency hybrid atmospheric water heater that overcomes many of the problems of current water heaters. Specifically, the present invention is capable of achieving approximately 99% efficiency, can be operated on a wide variety of fuel types, is capable of achieving low nitrogen oxides (NOx) emissions, can be built to any size, operates with consistent burner performance and low noise and vibration, and is constructed of economical materials.
2. Description of the Related Art
Applicant""s water heater is a combination of a direct contact water heater, i.e. a water heater where the combustion gases come into direct contact with the water that is being heated, and an indirect contact water heater, i.e. a water heater where the combustion gases are contained within a combustion chamber and exhaust tubes and do not come into direct contact with the water that is to be heated which is located on an opposite side of the combustion chamber wall and on an outside surface of the exhaust tubes.
The present invention combines a direct contact water heater portion and an indirect contact water heater portion to create a new and better type of water heater. Current water heater designs have three areas where improvement can be made. These areas where improvement is needed are in the location of the water inlet, the design of the combustion chamber, and the exit of the exhaust tubes.
The first problem with current water heater designs is in the location of the water inlet. The optimum location for the makeup water inlet in a direct contact water heater is at the top of the unit. By adding the cooler makeup water at the top of the unit, maximum heat exchange can occur between the downwardly cascading makeup water and the upwardly flowing combustion gases. Water heaters with the makeup water inlet located at the bottom of the unit are less efficient, able to achieve only approximately 91% efficiency as compared to approximately 99% heat transfer efficiency in the present invention.
Although makeup water to the heater is introduced through the makeup water inlet, the present invention is also provided with a separate water recirculation inlet at the top of the indirect contact water portion. This water recirculation inlet is for recirculating water within the lower indirect contact portion of the water heater whenever the temperature of the water to be reheated is greater than or equal to 150 degrees Fahrenheit. In the case of recirculation water temperatures that are less than 150 degrees Fahrenheit, the recirculation water is distributed by the upper makeup water inlet along with any makeup water that is needed.
Prior art water heaters teach a single water distribution location for water to be heated. This single water distribution location on prior art water heaters is always located at the top of the heating tower for both cold makeup water and for hotter recirculation water. Entry of hotter recirculation water to the upper portion of the water heaters, i.e. the direct contact portion, creates a high degree of vaporization of the distributed water stream. This vaporization cools down the water stream, making the overall efficiency of water heaters that move hotter recirculation water up to the top portions of the unit, i.e. the direct contact portion, lower than can be achieved when limiting the flow of hotter recirculation waters only to the lower sections, i.e. the indirection contact portions.
The second problem with current water heater designs is in the design of their combustion chamber. Prior art water heaters with the combustion chamber under the water line employ as their combustion chamber either a small diameter immersion tube or open bottom atmospheric combustion burner box instead of the large diameter, forced draft, firing chamber of the present invention. Both the small diameter firing tube and the open bottom atmospheric burner limit the amount of energy that can be burned to approximately 3 million BTUs/hr., thereby limiting the size of the water heater that can be produced when it employs one of these two types of combustion chamber. The present invention, on the other hand, employs a large diameter firing chamber that does not limit the size of the water heater unit with which it is employed. The present invention employs a forced draft combustion burner which allows efficient and clean combustion of liquid fuels. In addition to giving clean, efficient combustion, the firing chamber is large enough to allow the flame to be fully developed without impinging on any wall structure of the fire box. The combustion chamber of the present invention allows for adequate combustion space to burn large BTU energy releases of either gaseous or liquid fuels. Also, because the combustion chamber is circular when viewed from a top perspective, this makes the fire box more economical to produce than the square and rectangular prior art fire boxes and also minimizes the creation of thermal hot spots that produce NOx, thus making the present invention a lower NOx producing device than prior art rectangular fire box designs.
Also, both a small diameter firing tube and an open bottom atmospheric burner are not designed to produce low NOx emissions at the concentration of 20 ppm or less. Therefore, a water heater employing either of these two types of combustion chambers could not be used in California or Texas or anywhere that is designated by the EPA as a non-attainment zone and requiring 20 ppm NOx or less. The present invention, in contrast, is able to achieve low NOx emissions less than 20 ppm due to its use of a larger diameter firing chamber.
In addition, both a small diameter firing tube and an open bottom atmospheric burner can only be used with gaseous fuels, and neither can be employed with fuel oil or other combustible liquid fuel. The inability to burn oils severely limits the use of water heaters with these two types of combustion chambers, particularly in the eastern states in the United States and in foreign countries both of which are heavily dependent on liquid heating fuels. The present invention does not have this limitation because its large diameter firing chamber can easily burn either gaseous or liquid fuel sources. The large diameter firing chamber of the present invention has a plurality of exhaust tubes as the only exit out of the chamber for combustion gases.
The third problem with current water heater designs is in the exit of the exhaust tubes. Current gas fired water heaters, such as for example the one taught in U.S. Pat No. 4,658,803 to Ball et al., employ an overhead canopy or cover above the exit of the exhaust tubes. The purpose of the overhead canopy is to prevent water from entering the exhaust tubes. Exhaust tubes which have an overhead canopy experience two problems: overheating of the dry portion of the metal tubes located above the water line and below the canopy and sporadic backpressure created by a water curtain that is created by the canopy. Also the cost of manufacturing units with canopy style designs is higher than production of the present invention.
The immediate metal surfaces of the metal tubes, i.e. those portions of the metal tubes that are located above the water contained in the water chamber which surrounds the tubes and also located under the canopy where falling water cannot contact their exterior surfaces, remain dry and tend to overheat since they are not cooled by contact with the water. This results in overheating of the metal tubes and possible metal failure or at least greatly reduced tube life. One solution to this problem is to add an elaborate water cooling system to cool this portion of these metal tubes, however this type of cooling system is expensive to manufacture.
The solution that is normally employed to address this problem is to utilize expensive metal, i.e. special steel, which is capable of withstanding the elevated temperatures when manufacturing those dry portions of the metal tubes. Because of the expense of this special high temperature metal, in order to reduce costs, the tubes are generally produced in two sections, i.e. a lower wet section and an upper dry section. The lower wet section is constructed of less expensive metal since this section has the advantage of being contacted with water which dissipates heat from the wet surface. The upper dry section is constructed of the more expensive high temperature metal to withstand the high temperatures that result because this section is not cooled by water contact. However, use of two types of metals requires a splice or junction between the two sections which can be another place where leakage and metal failure can occur.
The present invention addresses this problem by employing an exhaust collector can into which the exhaust tubes attach at their exit. The exhaust tubes terminate into the exhaust collector can that allows for the exit of exhaust gases and also allows water to contact all exterior surfaces of the exhaust tubes and the exhaust collector can. Cascading water being sprayed either from the makeup water inlet or the recirculation inlet cools the metal, yet the exhaust collector still prevents water from entering the exhaust tubes. Thus because none of the metal surfaces experience extreme temperatures, the present invention can be constructed of lower cost metals, thereby keeping the construction cost low for this invention.
An alternate embodiment of the present invention employs a xe2x80x9ccandy canexe2x80x9d shape to the exhaust tubes instead of terminating the exhaust tubes in an exhaust collector can. Like the use of an exhaust collector can, use of exhaust tubes with a u shaped downwardly opening exit benefits from allowing all of the exhaust tube surfaces to be wet and creating natural pathways for air flow out of the tubes, thereby preventing the creation of back pressure pockets.
Still another problem with the use of a canopy located above the exit of the tubes is that this arrangement causes a continuous water curtain to be formed around the tubes where the water cascades over the canopy and falls unimpeded downward to the water surface of the water located in the water chamber that surrounds the tubes. Exhaust gases that exit the tubes are trapped under the water curtain until sufficient back pressure is created in the tubes and in the combustion chamber to blow the combustion gases through the water curtain. The resulting sporadic backpressure and periodic relief thereof causes inconsistent burner performance, and causes undesirable noise to emanate from the heater and causes destructive vibration within the water heater unit.
Space provided around the bottom of the exhaust collector can and between the tubes in the present invention creates natural air flow paths for exhaust gas to exit the collector can, thereby preventing buildup of backpressure and the resulting problems associated with creation and release of that backpressure.
The present invention is a hybrid atmospheric water heater that combines portions of a direct contact water heater and an indirect contact water heater within an outer shell to form a more efficient water heater.
The water heater is provided with an upper direct contact portion and a lower indirect contact portion. Makeup water or recirculation water that is less than 150 degrees Fahrenheit is introduced into the upper direct contact portion, and cooled exhaust gases exit the unit in the upper direct contact portion. A lower indirect contact portion is provided immediately underneath the upper direct contact portion. Below the lower direct contact portion, fuel is burned in a combustion chamber and exhaust tubes pass the hot combustion gases out of the combustion chamber through a water filled compartment of the indirect portion to the bottom of the direct contact portion of the heater. A burner fueled by either liquid or gaseous fuel fires into the large diameter combustion chamber. The combustion chamber is preferably provided with a fire wall that is either insulated with refractory or is alternately provided with a liquid cooled shell.
Indirect heat exchange occurs between the hot combustion gases and water located in the indirect or first heat transfer zone via a bottom plate that separates the combustion chamber from the water filled first heat transfer zone and via the exhaust tubes. The first heat transfer zone is the water compartment that surrounds the exhaust tubes. This water compartment is partially filled with water so that a lower portion of the exhaust tubes are constantly covered with water.
In addition to the indirect heat exchange that occurs through the bottom plate and the exhaust tubes, indirect heat exchange also occurs between the exhaust collector can into which the exhaust tubes exit and water that cascades down over the tubes and can. This cascading water comes from one of two sources. It either comes from the second heat transfer zone located directly above the first heat transfer zone, or alternately, when the second heat transfer zone is not in use, i.e. when the recirculation water to be heated is at or above 150 degrees Fahrenheit, the cascading water comes from recirculation water that cascades down over the tubes and can from a recirculation water spray nozzle provided on a water recirculation line.
The recirculation water spray nozzle receives water via a water pump from the bottom of the first heat transfer zone when the second heat transfer zone is not in use. Alternately, water drawn out of the bottom of the first heat transfer zone can be recirculated to a makeup water spray nozzle located at the top of the second heat transfer zone or pumped out of the unit for use. The makeup water spray nozzle also receives fresh makeup water via a makeup water line.
In an alternate embodiment, instead of employing a collection can at the exit of the exhaust tubes, alternate exhaust tubes are employed each of which is shaped in the form of a candy cane. In these alternate exhaust tubes, the exit for each tube is located in a shorter, downwardly extending portion of the candy cane-shaped exhaust tube.
Water from the makeup water spray nozzle cascades over a packed bed of heat exchange elements. The heat exchange elements facilitate heat transfer between the downwardly flowing water and the upwardly traveling exhaust gases. The exhaust gases exit the first heat transfer zone via exhaust tubes and the attached exhaust collector can, or alternately, via the alternate candy cane-shaped exhaust tubes.
Then, the exhaust gases enter the second heat transfer zone from the first heat transfer zone via a perforated packing shelf that separates the first and second heat transfer zones within the water heater. The heat exchange elements are supported by and prevented from entering the first heat exchange zone by the perforated packing shelf. However, the perforations in the perforated packing shelf freely allow water to pass downwardly through the shelf from the second heat exchange zone into the first heat exchange zone and also allow hot exhaust gases to pass upwardly through the shelf from the first heat exchange zone into the second heat exchange zone.